This is a circuit for allowing a fractional number of scans to be used to expose each data raster in a raster output scanner (ROS), and more specifically is a circuit for allowing a single scan to start with video data from one raster, and switch at any point to the data of the next raster.
In the simplest case a raster output scanned device has one scan per raster. An example is a scan generated by a rotating polygon which is used to scan an image onto a photoreceptor. At the start of scan (SOS) one line of video is clocked out and used to modulate the beam intensity to produce one raster. It is also possible to use two or more scans for each raster in this case the video for all of the scans in a raster is identical.
A problem arises when the scan and raster pixel densities required are not multiples of each other. Assume, for example, that a 240 spot per inch video generator is being used to drive a 350 spot per inch printer. In the scan direction, the clock speed of the video can be varied to match the scan speed to result in the proper image width. However, in the process direction, matching the video rate to the printer's number of scans per inch is a very difficult process. The number of scans per inch at the printer is normally not variable so the only method is to convert the video electronically, in this example, from 240 to 350 scans per inch. This can be done by converting the entire image to a continuous gray scale image using some numerical process, and then converting that continuous image into the desired 350 scan lines per inch. Of course, in the process, much computer time and memory is consumed, and there is inevitably a loss of detail.
A better method of making a fractional adjustment in the number of scan lines per inch is required.